Algorithm for discrimination of 1:1 tachycardias

ABSTRACT

An algorithm for detection of tachycardias and for discriminating between supraventricular tachycardia (SVT) and ventricular tachycardia (VT) when a 1:1 tachycardia condition is present that can be implemented in an implantable cardiac rhythm management device. Variability measures of AV and VA intervals during the tachycardia are computed and used to distinguish between SVT and VT.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.09/982,116, filed on Oct. 17, 2001, now issued as U.S. Pat. No.6,748,269, the specification of which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to methods and systems for treating cardiacarrhythmias. In particular, it deals with discriminating betweendifferent types of tachyarrhythmias.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidrate, typically expressed in units of beats per minute (bpm), that canoriginate in either the ventricles or the atria. Examples oftachyarrhythmias include sinus tachycardia, atrial tachycardia, atrialfibrillation, ventricular tachycardia, and ventricular fibrillation. Themost dangerous tachyarrythmias are those that have their origin in theventricles, namely ventricular tachycardia (VT) and ventricularfibrillation (VF). Ventricular rhythms occur when re-entry of adepolarizing wavefront in areas of the ventricular myocardium withdifferent conduction characteristics becomes self-sustaining or when anexcitatory focus in the ventricle usurps control of the heart rate fromthe normal physiological pacemaker of the heart, the sino-atrial node.The result is rapid contraction of the ventricles out ofelectromechanical synchrony with the atria. Most ventricular rhythmsexhibit an abnormal QRS complex in an electrocardiogram (ECG) becausethey do not use the specialized conduction system of the ventricles, thedepolarization spreading instead from the excitatory focus or point ofre-entry directly into the myocardium. In ventricular tachycardia, theventricles contract rapidly and produce distorted QRS complexes in anECG. Ventricular fibrillation, on the other hand, occurs when theventricles depolarize at an even more rapid rate and in a chaoticfashion, resulting in QRS complexes of constantly changing shape andvirtually no effective pumping action. Both ventricular tachycardia andventricular fibrillation are hemodynamically compromising, and both canbe life-threatening. Ventricular fibrillation, however, causescirculatory arrest within seconds and is the most common cause of suddencardiac death.

Cardiac rhythm management devices known as implantablecardioverter/defibrillators (ICDs) are designed to treat ventriculartachyarrhythmias by delivering an electrical shock pulse to the heart.Cardioversion and/or defibrillation can be used to terminate mosttachyarrhythmias, including VT and VF. The electric shock terminates thetachyarrhythmia by depolarizing all of the myocardium simultaneously andrendering it refractory.

Another type of electrical therapy for tachycardia is antitachycardiapacing (ATP). In ATP, the heart is competitively paced with one or morepacing pulses in an effort to interrupt the reentrant circuit causingthe tachycardia. ATP therapy can successfully treat VT, but it is noteffective in terminating VF. Modem ICDs incorporate ATP capability sothat ATP therapy can be delivered to the heart when a tachycardia isdetected. Although cardioversion/defibrillation will terminatetachycardia, it consumes a large amount of stored power from the batteryand results in patient discomfort owing to the high voltage of the shockpulses. It is desirable, therefore, for the ICD to use ATP to terminatea tachyarrhythmia whenever possible.

In most ICDs with ATP capability, VF is distinguished from VT usingrate-based criteria so that ATP or shock therapy can be delivered asappropriate. The heart rate is usually determined by measurement of thetime interval between successive ventricular depolarizations. A measuredheart rate is classified as a tachycardia when the rate is in a VT zone,defined as a range of rates above a tachycardia detection rate (TDR) butbelow a fibrillation detection rate (FDR). A measured heart rate abovethe FDR, on the other hand, is in the VF zone and is classified asfibrillation. In a typical device, a tachyarrhythmia with a heart ratein the VT zone is treated with ATP therapy in order to avoid anunnecessary painful shock to the patient, and a defibrillation shock isdelivered if the heart rate is in the VF zone or if ATP pacing fails toterminate a tachyarrhythmia in the VT zone.

As aforesaid, VT can be detected when the ventricular rate falls withinthe VT zone. A rapid ventricular rate in the VT zone, however, is notnecessarily due to VT but can also result from a tachyarrhythmia thatoriginates from above the ventricles. Such tachyarrhythmias are referredto as supraventricular tachycardias (SVT's) and include sinustachycardia, atrial tachycardia, and atrial fibrillation. The normalrhythmic impulse of the heart is first generated in pacemaker tissueknown as the sino-atrial (SA) node, spreads throughout the atria causingatrial contraction, and is then conducted to the atrioventricular (AV)node where the impulse is delayed before passing into the ventricles.The ventricles of a normal heart are then electrically stimulated byexcitation emanating from the AV node that spreads via specializedconduction pathways. An abnormal rhythm in the atria can thus betransmitted antegradely to the ventricles in patient whose AV conductionpathway is intact. Such an SVT is characterized by elevated rates inboth the atria and the ventricles. Elevated rates in both the atria andventricles can also occur with VT as well, however, due to retrogradeconduction of excitation from the ventricles to the atria. Suchretrograde conduction is possible in most people and confounds thediscrimination between VT and SVT based upon atrial and ventricularrates alone when both rates are similar, a condition known as aone-to-one or 1:1 tachycardia.

It is desirable for an ICD to differentiate between an SVT and a VT,however, both for reasons of efficacy and safety. ATP therapy deliveredto treat an SVT will not be effective and potentially could make mattersworse by triggering a ventricular arrhythmia. Also, although most ICD'scurrently on the market today are designed to treat onlytachyarrhythmias of ventricular origin, some newer designs are capableof treating atrial tachyarrhythmias as well. It is thus important for anICD to recognize that an elevated ventricular rate is due to an SVTrather than a VT so that either ventricular ATP therapy can be withheldor more appropriate therapy can be delivered. Conversely, because VT isgenerally a more serious condition, the ICD also needs to detect VT witha high degree of sensitivity so that therapy can be delivered promptly.

SUMMARY OF THE INVENTION

The present invention relates to an algorithm that can be implemented ina cardiac rhythm management device for tachycardia detection and fordiscriminating between a ventricular tachycardia and a supraventriculartachycardia when both the atrial and ventricular rates are elevated andwithin defined tachycardia ranges. The device detects tachycardias bydetecting atrial and ventricular senses corresponding to atrial andventricular depolarizations, respectively, and measuring the cyclelength between consecutive senses in each chamber. An AA intervalcorresponding to a cycle length between consecutive atrial senses, and aVV interval corresponding to a cycle length between consecutiveventricular senses, are both computed, preferably as a median or otherstatistic of a number of individual cycle lengths measured during a datacollection time window. Ventricular fibrillation (VF) is then detectedwhen the VV interval is below a VF threshold. If the VV interval iswithin a tachycardia range defined as above the VF threshold but below aVT threshold, and the AA interval is within normal limits, a ventriculartachycardia (VT) is detected. If the AA interval is within a tachycardiarange defined as below an SVT threshold, and the VV interval is withinnormal limits, a supraventricular tachycardia (SVT) is detected. A dualtachycardia is detected if the VV and AA intervals are both within theirrespective tachycardia ranges and differ by more than a specified dualtachycardia limit value. A dual tachycardia refers to condition in whichboth VT and SVT are present simultaneously. A dual tachycardia ispresumed when the atrial and ventricular rates are so different from oneanother that the atria and ventricles can be assumed to be independentlydriven. Since VT is the more serious condition, a dual tachycardia canbe regarded as a VT for treatment purposes.

If a 1:1 tachycardia condition is present, defined as when the AA and VVintervals are both within their tachycardia ranges and differ from oneanother by no more than some specified 1:1 limit value (e.g., apercentage or an absolute rate difference), the tachycardia may beeither an SVT or a VT. To distinguish between these possibilities, aninterval variability measure is employed. AV intervals corresponding toa cycle length between an atrial sense and a next occurring ventricularsense with no intervening atrial sense, and VA intervals correspondingto a cycle length between a ventricular sense and a next occurringatrial sense with no intervening ventricular sense are both collectedduring a time window. Variabilities are then computed for both the VAand AV intervals based upon their measured individual cycle lengthsduring a specified time window, where the variability measure ispreferably an average deviation calculated as the sum of the absolutevalue of the difference of each cycle length from the mean divided bythe number of cycle lengths in the time window. Other alternatives forthe variability measure include the variance of the cycle lengthsmeasured during the specified time window, the difference between themaximum and minimum cycle lengths measured during the specified timewindow after the exclusion of outlier values, a difference between anupper percentile value and a lower percentile value of the cycle lengthsmeasured during the specified time window after the exclusion of outliervalues, and a sum of consecutive cycle length differences measuredduring the time window.

SVTs and VTs are then distinguished based upon the relative variabilityof the VA and AV intervals. The algorithm discriminates between an SVTand a VT when a 1:1 tachycardia condition is present by detecting an SVTif the VA interval variability exceeds the AV interval variability anddetecting a VT if the AV interval variability exceeds the VA intervalvariability. A refinement to the algorithm comes from recognizing thatone particular type of SVT, atrioventricular nodal reentrant tachycardia(AVNRT) is characterized by near simultaneous excitation of the atriaand ventricles. Accordingly, when a 1:1 tachycardia condition is presentand when either the AV or VA interval is less than a specified AVNRTlimit value (e.g., 80 milliseconds), AVNRT may be detected regardless ofthe AV and VA interval variabilities.

The algorithm may be further refined to take advantage of the predictivevalue of the relative magnitudes of the AV and VA intervals during lowerrate 1:1 tachycardias. The AV and VV intervals may each be representedby a median or other statistic of a number of individual cycle lengthsmeasured during the data collection time window. Then, if the VVinterval is no is more than a specified ratebreak threshold value duringa 1:1 tachycardia, an SVT may be distinguished from a VT by detecting anSVT if the VA interval exceeds the AV interval and detecting a VT if theAV interval exceeds the VA interval, irrespective of the AV and VAinterval variabilities. Relative VA and AV interval magnitudes cannot bereliably used to distinguish SVT from VT in patients with first degreeAV block, however, owing to their slowed AV conduction velocities. Thealgorithm may therefore be further modified to not employ relative VAand AV interval magnitudes for VT/SVT discrimination in patients knownto have first degree AV block. In such patients, however, an additionalcriterion may employed that supercedes discrimination on the basis ofrelative AV and VA interval variability: a VT is detected if a measuredAV conduction time in the patient during a normal rhythm is less than aspecified AV block limit value and the AV interval during a tachycardiais greater than a specified tachycardia AV limit value. Exemplary valuesfor the specified AV block limit value and the specified tachycardialimit value are 270 and 300 milliseconds, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing diagram of a supraventricular rhythm.

FIG. 2 is a timing diagram of a ventricular rhythm.

FIG. 3 is a block diagram of a cardiac rhythm management device with ATPand cardioversion/defibrillation capability.

FIG. 4 is a flowchart of an exemplary implementation of the invention.

DETAILED DESCRIPTION

For the reasons stated above, accurate discrimination of 1:1tachycardias would be a very useful capability for an ICD. Much of theprevious research that has been done in this area has relied on signalmorphology to distinguish between SVTs and VTs where the morphologies ofthe individual electrogram waveforms are analyzed. Morphology basedmethods have shown promise, but these methods require computationalcomplexity that may be beyond the capability a small battery powereddevice such as an ICD. The present invention, on the other hand, is acomputationally efficient method for discrimination of 1:1 tachycardiasthat is relies upon the relative variability of measurable intervalsthat may be readily implemented in an ICD. In what follows, thealgorithm will be described in detail and its physiological basisexplained.

1. Physiological Basis

In normal sinus rhythm, which is a supraventricular rhythm, theelectrical impulse begins from spontaneous depolarization in thesinoatrial (SA) node. This impulse propagates through the atria to theatrioventricular (AV) node. The AV node introduces a delay, then thedepolarization signal continues down to activate the ventricles. Thetiming of this process is a function of the speed of signal propagationthrough the atrial tissue, AV node, and ventricles, and the distance ofsignal transmission in each region. This conduction pattern repeatsindefinitely. The timing as measured from atrial depolarization to thesubsequent ventricular depolarization, denoted the AV interval, isrelatively consistent from one beat to the next, but does reflect acertain amount of natural variance. This process is repeated at theinitiation of the next atrial impulse. The timing between successiveatrial depolarizations is determined by the intracellular activity ofionic currents, and is also subject to some variability, called the AAinterval variance. The VA interval of a normal cycle, then, is the timemeasured from ventricular depolarization to the next atrial impulse.There is no direct physical relationship between this ventricularimpulse and the next atrial impulse. The earlier ventricular impulse wasdetermined by the previous atrial impulse, and the next atrial impulseis determined by the electrochemical activity in the sinus node. So theVA interval can safely be viewed as merely the subtraction of the AVinterval from the VA interval—the remainder of the AA cycle. FIG. 1shows a timing diagram for a supraventricular rhythm illustrating theserelationships. The variability of the AV interval over severalsuccessive cycles can be measured with a variance calculation and notedas σ² _(AV) where AV is the random variable representing the AVintervals. The variance of the AA interval over several cycles canlikewise be calculated, designated as σ² _(AA). The VA interval (merelya mathematical derivation of the AV and AA intervals) has a varianceimposed upon it which is a function of the AV variance and the AAvariance. Since the AV interval is determined solely by atrioventricularconduction characteristics, and the AA interval is determined by theelectrochemical processes responsible for automaticity in the cells ofthe SA node, these two factors are assumed to be independent. The VAvariance σ² _(VA) is thus calculated as:σ² _(VA)=σ² _(AA)+σ² _(AV)While the AV variance has a value of σ² _(AV) and the AA variance avalue of σ² _(AA), the VA variance has a value of σ² _(AV)+σ² _(AA)which thus is larger than either σ² _(AV) or σ² _(AA) (assuming neitherof these quantities is zero). Thus, in a supraventricular arrhythmia theVA variance will be larger than the AV variance.

In a ventricular tachycardia conducting retrogradely through the AV nodeand depolarizing the atrium the reverse situation exists. The VAinterval represents the time interval from ventricular depolarizationbackwards through the AV node to atrial conduction. This sequence has aVA variability associated with it similar to the AV variance in theantegrade case. The ventricular chamber is now controlling the heartactivity so the ventricular to ventricular cycle (VV) is the drivingforce and has a VV interval variability governed by the arrhythmiamechanism on the cellular level. In this situation the AV interval ismerely the remainder of the cycle from atrial depolarization to the nextventricular depolarization and does not directly represent aphysiological event. A similar analysis can be done for the retrogradesituation as was previously shown for an antegrade situation. This timethe relevant random variables are VA and VV, and the variancesassociated with them are σ² _(VA) and σ² _(VV). Since the randomvariables VV and VA are independent, the variance of the AV interval inVT with retrograde conduction, σ² _(AV), is a function of the VAinterval variance and the VV interval variance and is calculated as:σ² _(AV)=σ² _(VV)+σ² _(VA)This implies the AV variance will be larger than either the VA or VVinterval variance during a retrogradely conducting arrhythmia. FIG. 2shows the timing diagram for this situation.

From this analysis, it can be seen that in an antegrade conductionsituation, σ² _(VA)=σ² _(AA)+σ² _(AV), and in a retrograde conductionsituation σ² _(AV)=σ² _(VV)+σ² _(VA). In both cases a comparison betweenσ² _(AV) and σ² _(VA) can be made. In an SVT, σ² _(VA) will always belarger than σ² _(AV) since it is equal to the sum of σ² _(AV) plusanother non-zero term. Likewise, in a VT, σ² _(AV) will always be largerthan σ² _(VA). The relative magnitudes of these two interval variancesis thus evidence of the direction of conduction of the arrhythmia.Therefore, if the AV interval variability is smaller than the VAinterval variability, a supraventricular rhythm is likely. Similarly, ifthe VA interval variability is smaller than the AV interval variability,a retrograde ventricular tachycardia can be predicted to be present.

2. Algorithm Description

An algorithm for detecting specific tachyarrhythmias that incorporates amethod for discrimination of 1:1 tachycardias based upon the relativevariability of VA and AV intervals will now be described. The algorithmis designed to diagnose the easily identifiable and non-1:1 rhythmsusing conventional rate-based criteria. If a rhythm does not meet any ofthese preliminary criteria, it is then considered for 1:1 discriminationusing the VA and AV interval variabilities.

The timing differences between atrial and ventricular sensesrepresenting depolarization events as detected by a conventional cardiacrhythm management device are used to calculate AA intervals (the timeinterval between consecutive atrial depolarizations), VV intervals (thetime interval between consecutive ventricular depolarizations), AVintervals (the time interval between an atrial depolarization and thenext ventricular depolarization—if one exists before the next atrialdepolarization), and VA intervals (the time interval between aventricular depolarization and the next atrial depolarization—if oneexists before the next ventricular depolarization). These data can becollected during a sliding data collection time window over a certainnumber of beats. The first x number of VV intervals are collectedtogether with as many AA, AV, and VA intervals as are included in thetime range. A presently preferred value for the number of VV intervalsto be contained within the time window is between 8 and 11. Thenecessary analysis for tachycardia detection as described below is thenperformed. After that, the next ventricular beat (and thus VV interval)is added to the end of the interval array and the first interval isdropped. At the same time, any AA, AV, or VA intervals that had timesequal to or before the dropped VV interval are also dropped and any newvalues within the next VV interval added. The sliding data collectiontime window thus acts in a manner similar to a first-in-first-out queueand allows the algorithm to continuously take into account new data.

The first thing an ICD must do for tachyarrhythmia detection isdetermine if the heart rate in either chamber is unusually fast. Forthis initial measurement, rate is computed by taking the median or otherstatistic of the individual cycle lengths for the beats included in thedata collection window. A median is preferably used because it is notstrongly influenced by possible long or short outliers. Outliers can bepresent for many reasons including missed or extra triggers andpremature complexes which are not part of the driving rhythm. If a fastrate is found in either chamber, then further analysis is performed. Ifthe ventricular rate is above the VF threshold, a VF is diagnosedregardless of what is happening in the rest of the heart. This is thefirst and most important condition to be checked. When a patient has anICD implanted, the physician makes an informed decision about theboundary between the rate threshold of ventricular fibrillation andventricular tachycardia for that patient. Any rhythm with a ventricularrate above the VF threshold is instantly diagnosed as VF to prevent anydelay or missed treatment for this lethal condition. An exemplary VFboundary is a 240 ms VV cycle length which corresponds to a ventricularrate of 250 bpm.

Next, the algorithm determines whether the rate in either or bothchambers falls in the fast, but non-VF range so as to constitute atachycardia or is normal, and the rhythm is further classified.Classification is accomplished with four mutually exclusive conditions.The possible combinations are: normal rate in both chambers, fast atrialrate and normal ventricular rate, fast ventricular rate and normalatrial rate, and fast rate in both chambers, where fast rate refers to arate within a tachycardia range defined for each chamber. If the rate isnot fast in either chamber, there is no reason to compute anythingfurther or deliver any therapy. If the rate is fast only in the atrium,then a supraventricular tachycardia can be diagnosed with certainty, andeither no therapy or atrial anti-arrhythmic therapy can be given. Ifonly the ventricular rate is fast, a ventricular tachycardia isdiagnosed, and ventricular ATP therapy can be delivered. The moredifficult situation arises when the rate is fast in both chambers. Ifthis is the case, it is necessary to determine if the rhythm is a 1:1tachycardia.

A 1:1 tachycardia is defined as when the AA and VV intervals are bothwithin their tachycardia ranges and differ from one another by no morethan some specified 1:1 limit value such as a value representing maximumpercentage of rate difference or an absolute rate difference. Forexample, a 1:1 tachycardia may be defined as the situation where theatrial and ventricular rates are in their tachycardia ranges and,additionally, where the rate in the atrium differs by no more than 10percent of the rate in the ventricles in either direction or where therate differs by no more than 10 beats per minute in either direction. Ifthe rate in both chambers is fast, but the 1:1 criterion is notsatisfied, a dual tachycardia is diagnosed. Since in a dual tachycardiathere is independent abnormal activity in both chambers, ventriculartachycardia should be detected, and VT treatment should be administeredto avert this potentially dangerous situation. If the 1:1 condition issatisfied, the algorithm advances to the next stage.

After a 1:1 tachycardia condition is detected, a check may next be madethat determines whether the rhythm is an atrioventricular nodalreentrant tachycardia (AVNRT). Although technically a supraventriculararrhythmia, AVNRT actually has conduction originating in the AV node andmoving upward to the atria and downward to the ventriclessimultaneously. Because of this unique propagation pattern, atrial andventricular depolarizations occur at close to the same time. Thiscontrasts with the situation where one chamber initiates activation inthe other chamber sequentially. An AVNRT can therefore be diagnosed whena 1:1 tachycardia condition is present and when either the AV or VAinterval is less than a specified AVNRT limit value, irrespective of theAV and VA interval variabilities. An exemplary AVNRT limit value of 80ms may be used on the premise that propagating AV or VA intervalconduction is not likely to be physiologically plausible at times below80 ms. Thus if either of the AV or VA intervals are below 80 ms, anAVNRT can be diagnosed as a supraventricular arrhythmia with no VTtherapy being delivered.

In the next stage, the AV and VA variabilities are calculated. Becauseoutlier values can have such a large effect on a variability measure,such outliers may first be eliminated before computation of thevariabilities. As mentioned earlier, outliers can arise from missed orextra triggers, premature complexes in either chamber, sudden changes inmorphology, or spurious far-field detections from the other chamber.Outliers can be identified by placing upper and lower bounds around themedian value for the AA, AV, VA, and VV intervals contained within adata collection window. If any value in the window occurs outside thesebounds, it is not considered in the variability calculation. A moreaccurate mean value can also be computed from these outlier-adjustedinterval arrays. The values for these boundaries can be determined bymultiplying the median value by a specified factor. Presently preferredvalues for factor to be used to determine the upper and lower bounds,respectively, are 1.5 and 0.5.

A standard measure of variability is the variance, which is computed forthe AV and VA intervals using the following equation:

${Variance} = \frac{\sum\limits_{i = 1}^{n}\left( {x_{i} - \mu} \right)^{2}}{\left( {n - 1} \right)}$where,

x_(i) are the interval values

μ is the mean interval value

n is the total number of intervals

Other variability measures are also possible, including: an averagedeviation calculated as the sum of the absolute value of the differenceof each cycle length from the mean divided by the number of cyclelengths in the time window, the difference between the maximum andminimum cycle lengths measured during the time window after theexclusion of outlier values, a difference between an upper percentilevalue and a lower percentile value of the cycle lengths measured duringthe time window after the exclusion of outlier values, and a sum ofconsecutive cycle length differences measured during the time window.Computation for average deviation is almost the same as the variancecalculation except that it removes the squared term and replaces it withan absolute value which makes it computationally less demanding. Theperformance in the algorithm of this metric and the variance variabilitymeasure have been determined to be similar and superior to that of theother variability measures. Average deviation is therefore a presentlypreferred variability measure and can be calculated as:

${{Average}\mspace{14mu}{Deviation}} = \frac{\sum\limits_{i = 1}^{n}{{x_{i} - \mu}}}{n}$where,

x_(i) are the interval values

μ is the mean interval value

n is the total number of intervals

However the variability measure is computed, a comparison is madebetween the AV and VA interval varibilities. If AV variability isgreater than VA variability, a VT is predicted. Likewise, if the VAvariability is greater than the AV variability, an SVT is predicted.

The performance of the algorithm may be further enhanced by takingadvantage of the predictive value of the relative magnitudes of the AVand VA intervals during lower rate 1:1 tachycardias. Using thiscriterion alone, the AV and VA intervals are compared and if the AVinterval is smaller than the VA interval, an SVT is diagnosed. If the VAinterval is smaller than the AV interval, a VT is diagnosed. Theinterval-only criterion is not robust physiologically and results in acompromised sensitivity to VT when used over a wide range of VV cyclelengths. It works better at longer VV cycle lengths, however, because inthat case, only one of the two intervals (AV or VA) will have a durationthat can be expected to plausibly represent conduction through the AVnode. In an SVT, the AV interval represents the conduction time of theelectrical impulse from the atrium to the ventricles. The VA interval ismerely the remainder of the cycle length (the subtraction of the AVinterval from the VV interval) and is thus highly rate dependent. Slowerrates will produce longer VA intervals, and faster rates will produceshorter VA intervals. For fast tachycardias, both the AV interval sizeand the VA interval size can be in normal expected ranges forconduction. There is a point, however, at which the rate of an SVTextends the VA interval to durations no longer realistic forventriculoatrial conduction. Here, the diagnosis of SVT isstraightforward because a VT with 1:1 retrograde conduction without AVblock is impossible at VA intervals that large. The same principle worksin reverse for VTs. So at long cycle lengths, or slower rates, theinterval only method is accurate.

The benefits of the interval-only method can be combined with thevariability method by diagnosing slower 1:1 tachycardias using theinterval criterion and faster 1:1 tachycardias using the variabilitycriterion. To distinguish fast and slow tachycardias for this purpose, aparameter called the ratebreak threshold is compared with the VVinterval. All 1:1 tachycardias with a VV cycle length greater than thisthreshold are diagnosed using the relative magnitudes of the AV and VAintervals. All 1:1 tachycardias with a VV cycle length less than thisthreshold are diagnosed using the relative variabilities of the AV andVA intervals. A presently preferred value for the ratebreak thresholdparameter is 340 milliseconds.

When a patient has first degree AV nodal block, the function of the AVnode is impaired and the AV conduction time is much longer than normal.Sometimes AV block can develop suddenly during a tachycardia, but oftenit is present and can be diagnosed during normal sinus rhythm. AV blockis diagnosed by a simple measurement of the AV interval during normalsinus rhythm. If the AV interval during normal sinus rhythm is greaterthan 200 milliseconds, the patient is said to have first degree AVblock. The implications of AV block to a discrimination algorithm whichuses interval information for diagnosis are substantial. Patients withfirst degree AV nodal block have an abnormally long AV interval duringSVTs which creates an unusually short VA interval for the given rate. Analgorithm which uses interval measurements is confounded by patientswith AV block since these patients present AV and VA intervals which arefar from normal.

With the present algorithm, if the ventricular rate is faster than theratebreak threshold so that diagnosis is made on the basis of relativeAV and VA interval variability and not on the basis of the magnitudes ofthose intervals, no problem is presented by patients with first degreeAV block. If the VV interval is greater than the ratebreak threshold,however, patients with first degree block may be misdiagnosed without afurther modification to the algorithm. Although AV block can beintermittent and hence undiagnosed, and can develop over time afterimplantation, a physician will generally know which patients havechronic AV block and can easily enter that information into a devicewhen programming the other required fields. In such patients identifiedto have first degree AV block, the modified algorithm does not employrelative magnitudes of the AV and VA intervals to diagnose 1:1tachycardias and uses only relative AV and VA interval variabilityinstead. An additional criterion, however, may also be used for patientswith first degree AV block: a VT is detected if a baseline measured AVconduction time in the patient is less than a specified AV block limitvalue and the AV interval during a tachycardial is greater than aspecified tachycardia AV limit value. Exemplary values for the specifiedAV block limit value and the specified tachycardia limit value are 270and 300 milliseconds, respectively. A common distinguishing feature ofVT patients with first degree AV block is that they have a normal AVinterval of greater than 200 milliseconds and a tachycardia AV intervalthat may increase to greater than 300 milliseconds. It is unlikely for apatient with first degree AV block to have an AV interval increase by asmuch as 30 milliseconds during an SVT. If a patient with first degree AVblock has a baseline measured AV interval of less than 270 ms whichincreases to more than 300 ms during a tachycardia, a ventriculartachycardia can be confidently diagnosed. Otherwise, the diagnosis ismade by AV and VA interval variability alone without using AV and VAinterval magnitudes regardless of the cycle length.

Further modifications may be made to implementation details of thealgorithm in order to improve its performance. For example, thealgorithm may make a rhythm diagnosis after looking at just one window,or it may require diagnosis information over several shifts of thewindow before making the diagnosis. In a presently preferred embodiment,the time window is shifted a specified number of times before making adecision. All of the diagnoses from each shift of the window are thenrequired to be identical before a final decision is made. If there isany disagreement or if a window within this time is not satisfied, thealgorithm continues until an agreement is reached over consecutivewindows shifted the specified number of times, or until a sustained highrate timer expires at which time VT therapy is given to ensure patientsafety. A presently preferred value for the specified number of requiredwindow shifts is four. Also, the algorithm may require all individualbeats within the window to demonstrate the cycle length required, or mayrequire some fraction of them to do so. A presently preferred embodimentis to require 100% of the beats in a window to demonstrate the cyclelength required for a diagnosis.

The algorithm may be further modified by adding a criterion to eliminatethe confusion that can occur during onset (or spontaneous conversion ifrelevant) of an arrhythmia. If AV and VA variability is being measuredto diagnose arrhythmias and some of the beats in the window aresupraventricular and non-arrhythmogenic (such as fast sinus rhythm) andsome of them due to a ventricular arrhythmia, their differentcharacteristics will increase the variability measure and produceconfused results. The window should ideally, of course, contain onlycycles of the arrhythmia of interest. The most common reason forcontaining other cycles is that the window has captured the onset ortermination of the arrhythmia along with some pre-arrhythmia orpost-arrhythmia data. To avoid this, the algorithm may require that boththe first and last cycles in the window be in the tachycardia raterange. This is further emphasized as the window is shifted because thefirst and last cycles must be at tachycardia rates for all shifts of thewindow.

3. Hardware Platform

In the description that follows, a microprocessor-based cardiac rhythmmanagement device will be referred to as incorporating the system andmethod that is the present invention. In the embodiment to be described,the invention is implemented with a control unit made up of amicroprocessor executing programmed instructions in memory. It should beappreciated, however, that certain functions of a cardiac rhythmmanagement device could be controlled by custom logic circuitry eitherin addition to or instead of a programmed microprocessor. The term“controller” as used herein should therefore be taken to encompasseither custom circuitry (i.e., dedicated hardware) or a microprocessorexecuting programmed instructions contained in a processor-readablestorage medium along with associated circuit elements.

Implantable cardiac rhythm management devices, such as pacemakers andICD's, are electronic devices that are implanted subcutaneously on apatient's chest with leads threaded intravenously into the heart toconnect the device to electrodes used for sensing electrical activityand for electrical stimulation of the heart. FIG. 3 is a system diagramof a microprocessor-based cardiac rhythm management device with thecapability of delivering cardioversion/defibrillation shocks as well asantitachycardia pacing (ATP) therapy. The device may also be configuredto deliver conventional (e.g., bradycardia) pacing as well. Thecontroller 10 of the pacemaker is a microprocessor that communicateswith a memory 12 via a bidirectional data bus. The memory 12 typicallycomprises a ROM (read-only memory) for program storage and a RAM(random-access memory) for data storage. The pacemaker has atrial andventricular sensing/pacing channels that respectively include electrodes24 and 34, leads 23 and 33, sensing amplifiers 21 and 31, pulsegenerators 22 and 32, and ventricular channel interfaces 20 and 30.Incorporated into each sensing/pacing channel is thus a pacing channelmade up of the pulse generator connected to the electrode and a sensingchannel made up of the sense amplifier connected to the electrode. Inthis embodiment, a single electrode is used for sensing and pacing ineach channel, known as a unipolar lead. Other embodiments may employbipolar leads that include two electrodes for outputting a pacing pulseand/or sensing intrinsic activity. The channel interfaces communicatebidirectionally with microprocessor 10 and include analog-to-digitalconverters for digitizing sensing signal inputs from the senseamplifiers and registers that can be written to by the microprocessor inorder to adjust the gain and threshold values for the sensingamplifiers, output pacing pulses, and change the pacing pulse amplitudeand/or duration. A telemetry interface 40 is also provided forcommunicating with an external programmer 500 that has an associateddisplay 510.

The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory, includingcontrolling the delivery of paces via the pacing channels, interpretingsense signals received from the sensing channels, and implementingtimers for defining escape intervals and sensory refractory periods. Thesensing circuitry of the pacemaker detects a chamber sense when a sensesignal (i.e., a voltage sensed by an electrode representing cardiacelectrical activity, sometimes called an electrogram signal) generatedby a particular channel exceeds a specified intrinsic detectionthreshold. A chamber sense may be either an atrial sense or aventricular sense depending on whether it occurs in the atrial orventricular sensing channel. Pacing algorithms used in particular pacingmodes employ such senses to trigger or inhibit pacing. By measuring theinterval between successive chamber senses, the controller is also ableto detect arrhythmias in the chamber based upon rate.

4. Exemplary Algorithm Implementation

FIG. 4 is a flowchart illustrating one particular embodiment of theSVT/VT discrimination algorithm as it could be implemented in the deviceof FIG. 3 by programming of the controller. The order of steps taken inthe algorithm is largely organized by the criticality of the rhythmsuspected. All of the metrics used are median or mean values computedover a single window of cycles. First, the atrial and ventricular ratesare measured at step T1 to determine if either is in the fasttachycardia zone. If neither of these rates is fast, nothing furtherneeds to be computed and the next cycle is obtained at step A1. Ifeither chamber demonstrates a fast rate, the next step is to determinewhether ventricular fibrillation (VF) is occurring. The condition usedto test this is a ventricular cycle length threshold. If the medianventricular cycle length is smaller than the VF threshold at step T2, VFis detected at step A2. If VF is not detected and the only chamberdemonstrating a fast rate is the ventricular chamber as determined atstep T3, ventricular tachycardia (VT) is detected at step A3. If theventricular chamber does not have a fast rate but the atrial chamberdoes as determined at step T4, supraventricular tachycardia (SVT) isdetected at step A4.

If none of the conditions have been satisfied thus far, a rhythm with afast rate in both chambers has been identified. The next step is todetermine if this rhythm is one-to-one (1:1). If the atrial rate doesnot differ by more than 10 beats per minute (bpm) in either direction ofthe ventricular rate at step T5, a 1:1 tachycardia is present.Otherwise, dual tachycardia (DT) is diagnosed at step A5 and VT therapyis recommended. If the rhythm is 1:1; a subset of SVTs, atrioventricularnodal reentrant tachycardias (AVNRT), can be diagnosed fairly easily bychecking if either the AV or VA interval is less than 80 ms at step T6.If so, SVT is diagnosed at step A6.

If the rhythm is not an AVNRT, the mean VV interval is compared to athreshold of 340 ms at step T7. If the VV interval is less than 340 ms,the average deviation of the AV and VA intervals is measured. If theaverage deviation of the AV interval is less than the average deviationof the VA interval at step T10, SVT is diagnosed at step A8. Otherwise,VT is diagnosed at step A7. If, however, the VV interval is greater than340 ms at step T7, the algorithm checks the normal AV interval size todetermine if the patient has first degree AV nodal block at step T8. Ifthe patient does not have AV block and the mean AV interval is less thanthe mean VA interval at step T11, SVT is detected at step A9. If thepatient does not have AV block and the mean AV interval is greater thanthe mean VA interval, VT is detected at step A7. If the patient doeshave AV block and the normal AV interval is less than 270 ms but thetachycardia interval is greater than 300 ms at step T9, VT is detectedat step A7. This condition is imposed to safely and easily address someof the VT cases without having to compute average deviation. If thepatient has AV block but the condition is not met at step T9, thepatient is diagnosed by average deviation by determining whether theaverage deviation of the AV interval is less than the average deviationof the VA interval at step T10. If it is, SVT is detected at step A8.Otherwise, VT is detected at step A7.

Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A cardiac rhythm management device, comprising: an atrial sensingchannel for detecting atrial senses corresponding to atrialdepolarizations; a ventricular sensing channel for detecting ventricularsenses corresponding to ventricular depolarizations; a controller fordetecting arrhythmias based upon the atrial and ventricular sensesdetected in the sensing channels, wherein the controller is programmedto: compute an AA interval corresponding to a cycle length betweenconsecutive atrial senses, a VV interval corresponding to a cycle lengthbetween consecutive ventricular senses, an AV interval corresponding toa cycle length between an atrial sense and a next occurring ventricularsense with no intervening atrial sense, and a VA interval correspondingto a cycle length between a ventricular sense and a next occurringatrial sense with no intervening ventricular sense; computevariabilities for both the VA and AV intervals based upon their measuredindividual cycle lengths during a specified time window; and, if a 1:1tachycardia condition is present, defined as when the AA and MVintervals are both within their tachycardia ranges and differ from oneanother by no more than a specified 1:1 limit value, discriminatebetween a supraventricular tachycardia and a ventricular tachycardiabased upon a relative variability of the VA and AV intervals.
 2. Thedevice of claim 1 wherein the AA, VV, VA, and AV intervals arestatistics computed from individual cycle lengths measured during a datacollection time window.
 3. The device of claim 2 wherein the AA, VV, VA,and AV intervals are medians of individual cycle lengths measured duringthe data collection time window.
 4. The device of claim 1 wherein thecontroller is further programmed to detect an atrioventricular nodalreentrant tachycardia (AVNRT) when a 1:1 tachycardia condition ispresent and when either the AV or VA interval is less than a specifiedAVNRT limit value, irrespective of the AV and VA interval variabilities.5. The device of claim 1 wherein the controller is further programmed todiscriminate between a supraventricular tachycardia and a ventriculartachycardia when a 1:1 tachycardia condition is present by: detecting asupraventricular tachycardia if the VA interval variability exceeds theAV interval variability; detecting a ventricular tachycardia if the AVinterval variability exceeds the VA interval variability.
 6. The deviceof claim 1 wherein the controller is further programmed to discriminatebetween a supraventricular tachycardia and a ventricular tachycardiawhen a 1:1 tachycardia condition is present and the VV interval is morethan a specified ratebreak threshold value, irrespective of the AV andVA interval variabilities, by: detecting a supraventricular tachycardiaif the VA interval exceeds the AV interval; and, detecting a ventriculartachycardia if the AV interval exceeds the VA interval.
 7. The device ofclaim 6 wherein the controller is further programmed to discriminatebetween a supraventricular tachycardia and a ventricular tachycardiawhen a 1:1 tachycardia condition is present based upon the relativevariability of the VA and AV intervals, irrespective of whether the VVinterval is more than the specified ratebreak threshold value, when apatient in whom the device is operating has an AV nodal block.
 8. Thedevice of claim 7 wherein the controller is further programmed todiscriminate between a supraventricular tachycardia and a ventriculartachycardia when a 1:1 tachycardia condition is present and the patientin whom the device is operating has an AV nodal block, irrespective ofthe relative variability of the VA and AV intervals, by detecting aventricular tachycardia if a measured AV conduction time in the patientis less than a specified AV block limit value and the AV interval isgreater than a specified tachycardia AV limit value.
 9. The device ofclaim 1 wherein the specified AV block limit value is 270 millisecondsand the specified tachycardia limit value is 300 milliseconds.
 10. Thedevice of claim 1 wherein the controller is further programmed tocompute each of the AV and VA interval variabilities as a measureselected from a group consisting of a variance of the cycle lengthsmeasured during the specified time window, a difference between themaximum and minimum cycle lengths measured during the specified timewindow after the exclusion of outlier values, a difference between anupper percentile value and a lower percentile value of the cycle lengthsmeasured during the specified time window after the exclusion of outliervalues, and an average deviation that is calculated as the sum of theabsolute value of the difference of each cycle length from the meandivided by the number of cycle lengths in the time window.
 11. A methodfor operating a cardiac rhythm management device, comprising: detectingatrial senses corresponding to atrial depolarizations; detectingventricular senses corresponding to ventricular depolarizations;computing an AA interval corresponding to a cycle length betweenconsecutive atrial senses, a VV interval corresponding to a cycle lengthbetween consecutive ventricular senses, an AV interval corresponding toa cycle length between an atrial sense and a next occurring ventricularsense with no intervening atrial sense, and a VA interval correspondingto a cycle length between a ventricular sense and a next occurringatrial sense with no intervening ventricular sense; computingvariabilities for both the VA and AV intervals based upon their measuredindividual cycle lengths during a specified time window; and, if a 1:1tachycardia condition is present, defined as when the AA and VVintervals are both within their tachycardia ranges and differ from oneanother by no more than a specified 1:1 limit value, discriminatingbetween a supraventricular tachycardia and a ventricular tachycardiabased upon a relative variability of the VA and AV intervals.
 12. Themethod of claim 11 wherein the AA, VV, VA, and AV intervals arestatistics computed from individual cycle lengths measured during a datacollection time window.
 13. The method of claim 12 wherein the AA, VV,VA, and AV intervals are medians of individual cycle lengths measuredduring the data collection time window.
 14. The method of claim 11further comprising detecting an atrioventricular nodal reentranttachycardia (AVNRT) when a 1:1 tachycardia condition is present and wheneither the AV or VA interval is less than a specified AVNRT limit value,irrespective of the AV and VA interval variabilities.
 15. The method ofclaim 11 further comprising discriminating between a supraventriculartachycardia and a ventricular tachycardia when a 1:1 tachycardiacondition is present by: detecting a supraventricular tachycardia if theVA interval variability exceeds the AV interval variability; detecting aventricular tachycardia if the AV interval variability exceeds the VAinterval variability.
 16. The method of claim 11 further comprisingdiscriminating between a supraventricular tachycardia and a ventriculartachycardia when a 1:1 tachycardia condition is present and the VVinterval is more than a specified ratebreak threshold value,irrespective of the AV and VA interval variabilities, by: detecting asupraventricular tachycardia if the VA interval exceeds the AV interval;and, detecting a ventricular tachycardia if the AV interval exceeds theVA interval.
 17. The method of claim 16 further comprisingdiscriminating between a supraventricular tachycardia and a ventriculartachycardia when a 1:1 tachycardia condition is present based upon therelative variability of the VA and AV intervals, irrespective of whetherthe VV interval is more than the specified ratebreak threshold value,when a patient in whom the device is operating has an AV nodal block.18. The method of claim 17 further comprising discriminating between asupraventricular tachycardia and a ventricular tachycardia when a 1:1tachycardia condition is present and the patient in whom the device isoperating has an AV nodal block, irrespective of the relativevariability of the VA and AV intervals, by detecting a ventriculartachycardia if a measured AV conduction time in the patient is less thana specified AV block limit value and the AV interval is greater than aspecified tachycardia AV limit value.
 19. The method of claim 18 whereinthe specified AV block limit value is 270 milliseconds and the specifiedtachycardia limit value is 300 milliseconds.
 20. The method of claim 11wherein the AV and VA interval variabilities are computed as variancesof their individual cycle lengths measured during the specified timewindow.
 21. The method of claim 11 wherein the AV and VA intervalvariabilities are each computed as a difference between the maximum andminimum cycle lengths measured during the specified time window afterthe exclusion of outlier values.
 22. The method of claim 11 wherein theAV and VA interval variabilities are each computed as a differencebetween an upper percentile value and a lower percentile value of thecycle lengths measured during the specified time window after theexclusion of outlier values.
 23. The method of claim 11 wherein the AVand VA interval variabilities are each computed as an average deviationthat is calculated as the sum of the absolute value of the difference ofeach cycle length from the mean divided by the number of cycle lengthsin the time window.
 24. The method of claim 11 wherein the AV and VAinterval variabilities are each computed as a sum of consecutive cyclelength differences measured during the time window.